Debt - Principal Terms of Senior and Junior Subordinated Notes (Details) $ in Millions	12 Months Ended
Dec. 31, 2020 USD ($)
Senior notes \| 2024 Senior Notes
Debt Instrument [Line Items]
Principal Amount at Maturity	$ 400.0
Stated coupon	4.25%
Redemption price percentage	100.00%
Senior notes \| 2024 Senior Notes \| Treasury Rate
Debt Instrument [Line Items]
Applicable basis spread over redemption price percentage	0.25%
Senior notes \| 2025 Senior Notes
Debt Instrument [Line Items]
Principal Amount at Maturity	$ 350.0
Stated coupon	3.50%
Redemption price percentage	100.00%
Senior notes \| 2025 Senior Notes \| Treasury Rate
Debt Instrument [Line Items]
Applicable basis spread over redemption price percentage	0.25%
Senior notes \| 2030 Senior Notes
Debt Instrument [Line Items]
Principal Amount at Maturity	$ 350.0
Stated coupon	3.30%
Redemption price percentage	100.00%
Senior notes \| 2030 Senior Notes \| Treasury Rate
Debt Instrument [Line Items]
Applicable basis spread over redemption price percentage	0.40%
Junior subordinated notes \| 2059 Junior Subordinated Notes
Debt Instrument [Line Items]
Principal Amount at Maturity	$ 300.0
Stated coupon	5.875%
Junior subordinated notes \| 2059 Junior Subordinated Notes \| On or after March 30. 2024
Debt Instrument [Line Items]
Redemption price percentage	100.00%
Junior subordinated notes \| 2059 Junior Subordinated Notes \| If changes in tax laws and interpretations occur
Debt Instrument [Line Items]
Redemption price percentage	100.00%
Junior subordinated notes \| 2059 Junior Subordinated Notes \| If a rating agency makes certain changes relating to the equity credit criteria for securities
Debt Instrument [Line Items]
Redemption price percentage	102.00%
Junior subordinated notes \| 2060 Junior Subordinated Notes
Debt Instrument [Line Items]
Principal Amount at Maturity	$ 275.0
Stated coupon	4.75%
Junior subordinated notes \| 2060 Junior Subordinated Notes \| On or after March 30. 2024
Debt Instrument [Line Items]
Redemption price percentage	100.00%
Junior subordinated notes \| 2060 Junior Subordinated Notes \| If changes in tax laws and interpretations occur
Debt Instrument [Line Items]
Redemption price percentage	100.00%
Junior subordinated notes \| 2060 Junior Subordinated Notes \| If a rating agency makes certain changes relating to the equity credit criteria for securities
Debt Instrument [Line Items]
Redemption price percentage	102.00%

Debt Instrument, Make Whole Premium, Discount Rate, Basis Spread on Variable Rate

No definition available.

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- Definition

Face (par) amount of debt instrument at time of issuance.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=119993939&loc=d3e28551-108399

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=114775985&loc=d3e28878-108400

+ Details

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- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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- Definition

Percentage price of original principal amount of debt at which debt can be redeemed by the issuer.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 470
-Section 50
-Paragraph 3
-Subparagraph e
-URI http://asc.fasb.org/extlink&oid=75038535&loc=d3e64711-112823

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